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Command Design Pattern in C++

Command design pattern

  1. Create a class that encapsulates some number of the following:
    • a "receiver" object
    • the method to invoke
    • the arguments to pass
  2. Instantiate an object for each "callback"
  3. Pass each object to its future "sender"
  4. When the sender is ready to callback to the receiver, it calls execute()
#include <iostream>  #include <string>  using namespace std;
class Person;

class Command
{
    // 1. Create a class that encapsulates an object and a member function
    // a pointer to a member function (the attribute's name is "method")
    Person *object; //    
    void(Person:: *method)();
  public:
    Command(Person *obj = 0, void(Person:: *meth)() = 0)
    {
        object = obj; // the argument's name is "meth"
        method = meth;
    }
    void execute()
    {
        (object-> *method)(); // invoke the method on the object
    }
};

class Person
{
    string name;

    // cmd is a "black box", it is a method invocation
    // promoted to "full object status"
    Command cmd; 
  public:
    Person(string n, Command c): cmd(c)
    {
        name = n;
    }
    void talk()
    {
        // "this" is the sender, cmd has the receiver
        cout << name << " is talking" << endl;
        cmd.execute(); // ask the "black box" to callback the receiver
    }
    void passOn()
    {
        cout << name << " is passing on" << endl;
        
        // 4. When the sender is ready to callback to the receiver,
        // it calls execute()
        cmd.execute(); 
    }
    void gossip()
    {
        cout << name << " is gossiping" << endl;
        cmd.execute();
    }
    void listen()
    {
        cout << name << " is listening" << endl;
    }
};

int main()
{
  // Fred will "execute" Barney which will result in a call to passOn()
  // Barney will "execute" Betty which will result in a call to gossip()
  // Betty will "execute" Wilma which will result in a call to listen()
  Person wilma("Wilma", Command());
  // 2. Instantiate an object for each "callback"
  // 3. Pass each object to its future "sender"
  Person betty("Betty", Command(&wilma, &Person::listen));
  Person barney("Barney", Command(&betty, &Person::gossip));
  Person fred("Fred", Command(&barney, &Person::passOn));
  fred.talk();
}

Output

Fred is talking
Barney is passing on
Betty is gossiping
Wilma is listening

Code examples

More info, diagrams and examples of the Command design pattern you can find on our new resource Refactoring.Guru.